This invention relates to high voltage circuit interrupters, and more specifically relates to a novel hybrid circuit interrupter consisting of a vacuum interrupter connected in series with an SF.sub.6 type interrupter with a varistor being connected in parallel with the vacuum interrupter to decrease the need for grading capacitors and to prevent the generation of high frequency discharges from the vacuum interrupter.
Interrupters consisting of the series connection of simultaneously opened diverse devices, particularly an SF.sub.6 interrupter and a vacuum interrupter, are well known, and are shown, for example, in the above-noted copending application Ser. No. 609,161, now U.S. Pat. No. 4,087,664, and in U.S. Pat. No. 3,982,088.
These devices combine the high dielectric strength of compressed SF.sub.6 (or interrupters using a similar electronegative gas or mixtures of electronegative gases), with the rapid dielectric recovery characteristics of a vacuum interrupter following a current zero. Thus, in the first few tens of microseconds following a current zero, the system recovery voltage is sustained by the vacuum interrupter. This period of time allows the proper deionization of the SF.sub.6 gas in the SF.sub.6 interrupter, so that the SF.sub.6 interrupter becomes capable of sustaining high voltage.
A problem has existed in the past of properly distributing the recovery voltage between the vacuum and SF.sub.6 interrupters. This can be obtained with parallel grading capacitors, but such capacitors for a 60 hertz system become excessively large.
Another problem with such systems is the tendency of the vacuum interrupter to display pre-discharges at high voltage levels before a full discharge occurs. These pre-discharges lead to high frequency currents in the system being protected, which can generate dangerous high frequency overvoltages due to the circuit inductance and capacitance. U.S. Pat. No. 3,982,088 proposes the mechanical reclosing of the vacuum interrupter after 2 to 20 cycles. This, however, does not elimate high frequency oscillation during the most critical time, shortly after interruption.